Characterization of microstructures across the heat-affected zone of the modified 9Cr-1mo weld joint to understand its role in promoting type IV cracking

In the postweld heat-treated (PWHT) fusion welded modified 9Cr-1Mo steel joint, a soft zone was identified at the outer edge of the heat-affected zone (HAZ) of the base metal adjacent to the deposited weld metal. Hardness and tensile tests were performed on the base metal subjected to soaking for 5...

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Bibliographic Details
Published in:Metallurgical and materials transactions. A, Physical metallurgy and materials science Physical metallurgy and materials science, 2007, Vol.38 (1), p.58-68
Main Authors: LAHA, K, CHANDRAVATHI, K. S, PARAMESWARAN, P, BHANU SANKARA RAO, K, MANNAN, S. L
Format: Article
Language:English
Subjects:
Alloys
Applied sciences
Atoms & subatomic particles
Carbides
Chemicals
Chromium steel
Creep tests
Exact sciences and technology
Grain boundaries
Grain growth
Grain size
Joining, thermal cutting: metallurgical aspects
Metallurgy
Metals. Metallurgy
Microstructure
Nuclear power plants
Scanning electron microscopy
Solid solutions
Stress corrosion cracking
Welding
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Summary:In the postweld heat-treated (PWHT) fusion welded modified 9Cr-1Mo steel joint, a soft zone was identified at the outer edge of the heat-affected zone (HAZ) of the base metal adjacent to the deposited weld metal. Hardness and tensile tests were performed on the base metal subjected to soaking for 5 minutes at temperatures below Ac^sub 1^ to above Ac^sub 3^ and tempering at the PWHT condition. These tests indicated that the soft zone in the weld joint corresponds to the intercritical region of HAZ. Creep tests were conducted on the base metal and cross weld joint. At relatively lower stresses and higher test temperatures, the weld joint possessed lower creep rupture life than the base metal, and the difference in creep rupture life increased with the decrease in stress and increase in temperature. Preferential accumulation of creep deformation coupled with extensive creep cavitation in the intercritical region of HAZ led to the premature failure of the weld joint in the intercritical region of the HAZ, commonly known as type IV cracking. The microstructures across the HAZ of the weld joint have been characterized to understand the role of microstructure in promoting type IV cracking. Strength reduction in the intercritical HAZ of the joint resulted from the combined effects of coarsening of dislocation substructures and precipitates. Constrained deformation of the soft intercritical HAZ sandwich between relatively stronger constitutes of the joint induced creep cavitation in the soft zone resulting in premature failure. [PUBLICATION ABSTRACT]
ISSN:1073-5623
1543-1940
DOI:10.1007/s11661-006-9050-0